A balun circuit that provides a single terminal signal port and a balanced signal port is known (refer to, for example, Patent Document 1). A first conductive element extends between the single terminal signal port and the ground. A second conductive element is coupled to the first conductive element like a transformer and electrically insulated from the first conductive element. The second conductive element electrically symmetrically extends from the ground to the balanced port. The first and second conductive elements are arranged opposed to each other across an electrical insulating layer. The balanced port is located electrically at the center of the second conductive element. The first conductive element is arranged such that the center point of the first conductive element, namely, the point defined as an electrical center of the first conductive element is opposed to the balanced port, and thereby be operable at a plurality of frequencies.
Further, there is a known high-frequency power amplifier having a pair of power amplifier elements that receive from the outside a first distribution signal and a second distribution signal distributed to have the same amplitude and the characteristics in opposite phases to each other and amplify and output the power of the received first distribution signal and second distribution signal (refer to, for example, Patent Document 2). A pair of transmission lines correspondingly connected to the pair of power amplifier elements are provided. The pair of transmission lines have a pair of bulging parts provided on side parts opposed to each other. The pair of bulging parts are opposed to each other at a space therebetween to constitute a capacitor.
Further, there is a known differential type distributed constant amplifier in which a grounded-source transistor and a grounded-gate transistor are cascode-connected (refer to, for example, Patent Document 3). The inductor suppresses reflection at a branch point of a distributed constant line on the input side.
[Patent Document 1] Japanese Examined Patent Application Publication No. 8-28607
[Patent Document 2] Japanese Laid-open Patent Publication No. 2002-252529
[Patent Document 3] Japanese Laid-open Patent Publication No. 2004-289554
An amplifier circuit is needed which can amplify a high-frequency signal with large output power and high gain. To increase the output power, it is necessary to increase the size of the transistor. However, increasing the size of the transistor decreases the gain of the high-frequency signal.